In the United States, it is estimated that approximately five million people have congestive heart failure (CHF). During treatment of some CHF patients, the contractility of the heart may be assessed for diagnosis and/or treatment purposes. In addition to being useful for generally monitoring the progression of a patient's cardiac disease, the heart's contractility may be monitored over time to monitor the patient's response to therapy and make any appropriate changes thereto. For example, some patients suffering from CHF have an abnormality in the electrical conducting system of the heart, sometimes referred to as “intraventricular conduction delay” or “bundle branch block” that causes the left and right ventricles to beat out of phase instead of simultaneously. Cardiac resynchronization therapy (CRT), sometimes referred to as “biventricular pacing”, may be used to re-coordinate the beating of the left and right ventricles by pacing both ventricles simultaneously. In contrast to CRT, conventional pacemakers typically only pace the right ventricle. During treatment of CRT patients, the systolic and diastolic pump properties of one or both of the ventricles may be assessed to assist pacing the ventricles and/or monitoring the heart's contractility over time.
Some known methods of assessing the heart's contractility include introducing a conventional blood pressure or pressure-volume loop transducer into the ventricle chamber from the femoral artery to determine the ventricular blood pressure and volume. However, such transducers are typically removed after each measurement because the presence of the transducer and associated components within the left ventricle risk causing a stroke. Accordingly, conventional transducers are generally not implantable within the left ventricle for continuously monitoring heart contractility over time. Medical imaging, such as echo imaging or magnetic resonance imaging (MIR), can be used to non-invasively measure ventricular blood pressure. However, medical imaging procedures are typically lengthy and expensive, and therefore may not be suitable for monitoring cardiac contractility over time because of the cost and/or inconvenience of the multiple of procedures to the patient.
Conventional diaphragm-type sensors (force gauges) may be implantable within the ventricles or within the pericardial space adjacent the ventricles for continuously monitoring contractility over time. However, conventional diaphragm-type sensors may need to be hermetically sealed to operate within the human body, and are typically battery-powered. Conventional diaphragm-type sensors may therefore be bulkier and/or less reliable than is desired for implantation within or adjacent to the heart. Moreover, the battery may limit the duration for which conventional diaphragm-type sensors may remain operable within the human body without being serviced or replaced.
Sensors fabricated from lead zirconate titnate (PZT), a piezoelectric ceramic material, have been contemplated for use as an implantable sensor that measures heart contractility. However, because PZT has a relatively large d33 coefficient, PZT sensors are typically sensitive to hydrostatic pressure and sound waves. Specifically, hydrostatic pressure and/or sound waves may cause the PZT sensor to respond to movement in a direction approximately parallel to a thickness of the PZT sensor. The response caused by such movement may add undesirable noise to the measurement signal of the PZT sensor that represents the motion of the heart, thereby reducing an overall signal clarity of heart contractility information. The relatively large d33 coefficient of PZT causes PZT to be sensitive to triboelectric charges generated within the PZT sensor by friction between the PZT sensor and surfaces with which the PZT sensor is in contact, thereby further reducing signal clarity of the heart contractility information. Moreover, because PZT contains lead, PZT may not be biocompatible and therefore not be suitable for implantation within the human body.
A need remains for an implantable sensor that is directed to overcoming one or more of the problems set forth above. A need remains for an implantable sensor with improved signal clarity that is biocompatible and does not require an external power source and/or hermetic sealing.